Mass flow meter systems and methods

ABSTRACT

A flow meter system that calculates mass flow rate based only on a single pressure signal. A flow controller is arranged in parallel with a restriction such that a constant pressure differential is maintained across the restriction. The pressure, and temperature if not controlled, of the fluid flowing through the restriction is measured on either side of the restriction. The pressure is compared to a plot of pressure versus mass flow rate calculated for the specific restriction and fluid being measured. The constant pressure differential maintained across the restriction yields a linear relationship between pressure and flow rate. If temperature is not controlled, the plot of pressure versus mass flow rate will remain linear, but the slope of the curve will be adjusted based on the temperature of the fluid.

This is a Divisional application of U.S. Ser. No. 10/081,174 filed Feb.21, 2002, which is based on U.S. Provisional Application Ser. No.60/283,596 filed Apr. 13, 2001. Both U.S. Ser. No. 10/081,174 and No.60/283,596 are fully incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to systems and methods for measuring andcontrolling mass flow and, more specifically, to such systems andmethods that allow precise measurement of mass flow using a flowrestriction and pressure and temperature sensors.

BACKGROUND OF THE INVENTION

In many disciplines, the mass flow of a fluid must be measured with ahigh degree of accuracy. For example, in medical and semi-conductormanufacturing, gasses and liquids often need to be delivered in precisequantities to obtain desired results. Meters are used to measure themass of the fluid actually delivered.

Conventional pressure-based mass flow meters employ a flow restriction,a temperature sensor, and pressure sensors for detecting the absolutepressure upstream of the flow restriction as well as the differentialpressure across the flow restriction. Mass flow is determined from atable that correlates the pressure and temperature readings withpredetermined mass flow rates. Such systems require at least twopressure sensors and a temperature sensor to account for fluid density,fluid velocity, and fluid viscosity under different temperatures andupstream and downstream pressures.

The need exists for mass flow meters that are simpler and require lesscomplex calculations to determine true mass flow.

RELATED ART

U.S. Pat. No. 5,791,369 to Nishino et al. discloses a flow ratecontroller that, purportedly, requires only one functional pressuretransducer. However, the controller disclosed in the '369 patentoperates only in the sonic flow regime, and this system requires thatthe inlet pressure be twice the outlet pressure for the controller tofunction properly. The flow controller of the '369 patent thus operatesonly with very low flow rates, only with gases, and must have effectivepressure regulation upstream. In addition, the '369 patent discloses theuse of a second pressure transducer to determine when the downstreampressure is more than half of the inlet pressure, and the controllershuts down when this condition is met.

U.S. Pat. No. 6,152,162 to Balazy et al. discloses a fluid flowcontroller that requires two pressure measurements, one upstream and onedownstream of a flow restrictor. The '162 patent does not measure massflow. The '162 patent also employs a filter element as the flowrestriction. Particles in the gas stream can clog the filter, therebychanging the relationship of pressure drop and flow characteristics oftheir flow restriction and possibly deviating from the initialcalibration setting.

U.S. Pat. No. 6,138,708 to Waldbusser discloses a pressure compensatedmass flow controller. The system described in the '708 patent combines athermal mass flow controller with a thermal meter coupled to adome-loaded pressure regulator. Another pilot pressure regulator usingan independent gas source loads the dome of the pressure regulatorupstream of the thermal mass flow controller. The pilot regulator andthe mass flow controller are controlled by a microprocessor so thatinlet pressure is controlled in concert with the flow rate resulting inan inlet pressure independent flow controller.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram depicting an exemplary mass flow meter of thepresent invention;

FIG. 2 is an exemplary plot of mass flow through the meter versus fluidpressure illustrating the operation of the present invention;

FIG. 3 is a somewhat schematic section view depicting an exemplarymechanical system that may be used to implement a mass flow meter asdepicted in FIG. 1;

FIG. 4 is a block diagram of a meter circuit employed by the mass flowmeter of FIG. 1;

FIG. 5 is a detailed block diagram of an exemplary meter circuit thatmay be employed by a mass flow meter employing the principles of thepresent invention;

FIG. 6 is a flow diagram representing one exemplary method ofcalibrating the mass flow meter of FIG. 1;

FIG. 7 is a plot of mass flow through the meter versus fluid pressurefor several different fluid temperatures illustrating compensation fordifferent fluid temperatures;

FIG. 8 is an exemplary plot of mass flow through the meter versus fluidpressure illustrating the basic principles of operation of the presentinvention applied to a non-linear mass flow output;

FIG. 9 is a block diagram of an exemplary flow control system employinga mass flow meter system of the present invention; and

FIG. 10 is a block diagram of an alternate embodiment of a flow controlsystem.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following discussion is organized in a number of sections. In thefirst section, the basic operation and theory of the present inventionwill be described in the context of a mass flow meter system. The secondand third sections will describe exemplary mechanical and electricalsystems that may be used to implement the present invention. The fourthsection will describe one method of calibrating a mass flow meter systemconstructed in accordance with the principles of the present invention.The fifth section describes the mass flow meter described in the firstthrough fourth section used as part of a mass flow controller. The sixthsection describes an alternate embodiment of a mass flow control system.The final section describes additional considerations that are typicallytaken into account when designing and constructing a particularimplementation of the present invention.

1. Mass Flow Meter System

Referring initially to FIG. 1 of the drawing, depicted at 20 therein isan exemplary mass flow meter system constructed in accordance with, andembodying, the principles of the present invention. The meter system 20comprises a mechanical system 22 and an electrical system 24. Themechanical system 22 comprises a flow restrictor 30 defining arestriction chamber 32 and a pressure balancing system 34. Theelectrical system 24 comprises a pressure sensor 40, a temperaturesensor 42, and a meter circuit 44.

The mechanical system 22 defines a fluid inlet 50 and a fluid outlet 52.The inlet 50 and outlet 52 are connected to a source or supply 54 ofpressurized fluid and a destination 56 of that fluid, respectively.

From the discussion above, it should be apparent that particulars of thesource 54 and destination 56 may vary significantly depending upon theenvironment in which the meter system 20 is used. For example, in amedical environment, the source 54 may be a bottle of pressurized gasand the destination 56 may be a mixer that mixes the gas with air anddelivered the mixture to a patient using conventional means. In amanufacturing environment, the source 54 may be a converter thatgenerates a supply of gas from raw materials and the destination 56 maybe a reaction chamber in which the gas is used as part of an industrialprocess. In many cases, the supply pressure at the source 54 and backpressure at the destination 56 may be unknown and/or variable.

The meter system 20 of the present invention is thus intended to be usedas part of a larger system in which pressurized fluid thus flows fromthe source 54 to the destination 56 through the mechanical system 22.The pressure balancing system 34 maintains a constant differentialpressure across the restriction chamber 32.

As depicted in FIG. 1, the exemplary flow restrictor 30 is variable. Inparticular, when the meter system 20 is calibrated the flow restrictor30 defines a predetermined geometry and an effective cross-sectionalarea of the restriction chamber 32. In the exemplary system 20, the flowrestrictor 30 may be changed to alter the geometry, and in particularthe effective cross-sectional area, of the restriction chamber 32. Inother embodiments of the present invention, the flow restrictor 30 neednot be variable, but instead can be fabricated with a preset geometryand effective cross-sectional area. This may or may not include astandard orifice, sonic orifice, laminar flow element of variousgeometries, or a variable area restriction. The use of a preset orvariable flow restrictor may affect the process of calibrating the metersystem 20 as will be discussed below.

The pressure balancing system 34 is preferably a flow controller thatutilizes a mechanical regulation system to maintain a constantdifferential pressure across the restriction chamber 32 even if thesource and destination pressures are unknown or variable. Suchmechanical flow controllers are disclosed, for example, in U.S. Pat. No.6,026,849 issued Dec. 2, 1999, and copending U.S. patent applicationSer. No. 09/805,708 filed Mar. 13, 2001 and commonly assigned with thepresent application. However, the pressure balancing system 34 may alsobe an electro-mechanical flow controller as disclosed in the '708application. The teachings of the '849 patent and the '708 applicationare incorporated herein by reference.

The pressure and temperature sensors 40 and 42 are preferablyelectro-mechanical transducers that convert pressure and temperaturevalues into an electrical signal. These sensors 40 and 42 areoperatively connected to the mechanical system 22 to generate electricalsignals indicative of the pressure and temperature, respectively, of thefluid flowing through the mechanical system 22.

The meter circuit 44 stores or otherwise has access to calibration datarelating mass flow rate to pressure and temperature for a given fluid.The calibration data includes a calibration factor calculated for agiven restrictor 30 and a gas constant determined by the characteristicsof the gas flowing through the meter system 20. The gas constant isbased on the specific gas density or viscosity as related to temperaturechanges.

Based on the calibration data and the pressure and temperature signals,the meter circuit 44 generates a flow output signal corresponding to themass flow of fluid through the mechanical system 22. The flow outputsignal may be recorded or displayed or used as part of a larger circuitfor controlling fluid flow from the source 54 to the destination 56.

Referring now to FIG. 2, depicted therein at 60 is a plot of pressureversus mass flow through the restrictor 30 when the pressure balancingsystem 34 is connected across the restrictor 30 as described above. Asseen in the figure, the mass flow output increases linearly with outletpressure. This curve 60 is an effect of the ideal gas law, which relatesvolume, mass, temperature, and non-linear compressibility effectstogether as described by the following equation (1):

PV=mRTZ  (1)

Where:

P=pressure,

m=mass,

V=volume,

R=Gas Constant (Universal),

T=temperature, and

Z=gas compressibility

 (in the following discussion, a “.” above any of these symbols denotesa mass of volume flow rate)

Dividing both sides of the ideal gas law equation by time yields thefollowing rate equation (2):

P{dot over (V)}={dot over (m)}RTZ  (2)

Solving for the rate equation (2) for mass flow rate yields thefollowing mass flow rate equation (3): $\begin{matrix}{\overset{.}{m} = {\frac{\overset{.}{V}}{R\quad T\quad Z}P}} & (3)\end{matrix}$

Rearranging the terms of the mass flow rate equation yields thefollowing slope equation (4):$\frac{\Delta \overset{.}{m}}{\Delta \quad P} = {\frac{\overset{.}{V}}{R\quad T\quad Z} = {\begin{matrix}{{s\quad l\quad o\quad p\quad e\quad o\quad f\quad t\quad h\quad e\quad l\quad i\quad n\quad e\quad a\quad r}\quad} \\{p\quad o\quad r\quad t\quad i\quad o\quad n\quad o\quad f\quad t\quad h\quad e\quad p\quad l\quad o\quad t}\end{matrix} = {C\quad O\quad N\quad S\quad T\quad A\quad N\quad T}}}$

The slope of equation (4) illustrates the relationship between the massflow rate and pressure for a given system and gas. If the pressureincreases, the amount of mass within a certain volume (i.e., density)will increase proportionally if temperature remains constant.Experimental data showed that the temperature only varied by a fractionof a degree throughout the entire experiment. Since the slope of theplot remained constant, the end result was the volumetric flow rate forthis device remained constant through the entire pressure range untilthe pressure differential pressure (i.e., inlet pressure minus outletpressure) approached a critical value.

In contrast, traditional flow meter devices that rely on pressuremeasurements must take into account three factors: inlet pressure, inlettemperature, and pressure differential across an orifice. The flow rateacross an orifice, or similar flow restriction, is expressed in generalterms by the following flow rate equation (5): $\begin{matrix}{\overset{.}{m}{{K\left\lbrack {d^{2}\frac{1}{{\sqrt{1\left( \frac{d}{D} \right)}}^{\quad 4}}} \right\rbrack}\frac{\sqrt{{Gp}_{1}}}{Z\quad T_{1}}\sqrt{p_{1} - p_{2}}}} & (5)\end{matrix}$

Where:

p₁=gas pressure upstream of the restriction

p₂=gas pressure downstream of restriction

T₁=temperature upstream of restriction

D=flow passage diameter

d=restriction hydraulic diameter (effective flow diameter)

G=specific gravity or normalized molecular weight of gas

Z=compressibility factor of gas

The term K in this flow rate equation (5) is a factor that is determinedexperimentally during the calibration of a given restriction. The term Kis dependent on the geometry of the restriction and expansion factors ofthe gas such as Joule-Thompson cooling/heating (i.e., the change intemperature caused by a sudden change in pressure). The flow rateequation (5) is only valid for low flows or restrictions that do notcreate large gas velocities inside of them. When the speed of the gasapproaches the speed of sound, the bulk speed of the gas molecules islarger than the speed at which pressure can travel through the medium.The flow properties take on significantly different relationships andare called compressible flows, sonic flows, or choked flows.

Therefore, traditional flow controllers relying on pressure drops employtwo pressure sensors and a temperature sensor. Such traditional flowcontrollers must also have relatively sophisticated electronics capableof calculating flow rate by measuring both pressures, calculating thepressure difference (with custom op amps (analog) or by means of aprogrammed digital microprocessor and the needed analog to digitalconverters), and most importantly, by calibrating the device to find outthe term K.

With the approach of the present invention, the equation to solve toobtain flow would look like one the following equations (6) or (7):$\begin{matrix}{{\overset{.}{m} = {\frac{K}{R}\frac{P_{1}}{T_{1}}\left( {{for}\quad a\quad {laminar}\quad {flow}\quad {restriction}} \right)}}{or}} & (6) \\{\overset{.}{m} = {\frac{K}{R}\frac{\sqrt{P_{1}}}{T_{1}}\left( {{for}\quad {an}\quad {orifice}\quad {type}\quad {restriction}} \right)}} & (7)\end{matrix}$

Where: K is the calibration factor and is determined during calibrationas will be discussed below. It should be noted that the constant, R, inEquations (6-10) is not the Universal constant, R, of Equations (1-4).Rather, it is a gas-dependent constant that varies for laminar flow ororifice type restrictions.

The case of some gases where non-ideal compressibility must be takeninto account, the following equations (8) and (9) may be used:$\begin{matrix}{{\overset{.}{m} = {\frac{K\quad P_{1}}{R\quad T_{1}}\left( \frac{1}{Z\left( {P,T} \right)} \right)\quad \left( {{for}\quad a\quad {laminar}\quad {flow}\quad {restriction}} \right)}}{or}} & (8) \\{\overset{.}{m} = {\frac{K\sqrt{P_{1}}}{R\quad T_{1}}\left( \frac{1}{Z\left( {P,T} \right)} \right)\quad \left( {{for}\quad {an}\quad {orifice}\quad {type}\quad {restriction}} \right)}} & (9)\end{matrix}$

Where: Z(P,T) is the compressibility factor that is dependent onpressure and temperature.

As can be seen by a comparison of equation (5) with any of the equations(6), (7), (8), or (9), the present invention greatly simplifies therelationship of mass flow rate to pressure and temperature. In mostcases, compressibility does not need to be obtained directly, becausethe controller will be calibrated such that compressibility is accountedfor in the calibration sequence.

At a given temperature and pressure, the gas may already be showing somenon-ideal compressibility that will be inherent in the measurement takenby the flow standard during calibration. In addition, the term R is gasspecific, so only the gas specific constant needs to be entered beforeor during calibration to have a highly accurate mass flow measurement.The calibration sequence may be implemented as will be described belowwith reference to FIG. 5.

After the calibration factor K is calculated using the calibrationsequence, mass flow may be measured using only the following linearslope equation (10) defining the slope of the plot 60 depicted in FIG.2:

Y=mx+b  (10)

Where: y=mass flow, x=measured pressure, m=K/RT, and b is the zerooffset.

With the foregoing basic understanding of the meter system 20 in mind,the various components of this system will now be described in furtherdetail below.

II. Mechanical System

Referring now to FIG. 3 of the drawing, depicted in detail therein isthe mechanical system 22 of the exemplary flow meter system 20. Therestrictor 30 of the mechanical system 22 is formed by a main bodyassembly 120. The main body assembly 120 defines a main passageway 130having an inlet 132 and an outlet 134 and defining the restrictionchamber 32. The restriction chamber 32 is arranged between the inlet 132and the outlet 134.

As generally discussed above, the mass flow meter system 20 measures themass flow of fluid that flows through the main passageway 130 from theinlet 132 to the outlet 134 using pressure and temperature signalsgenerated by the pressure sensor 40 and the temperature sensor 42. Thefluid flowing through the meter system 20 will be referred to herein asthe metered fluid. Seals are formed at the junctures of the variousparts forming the mechanical system 22 such that metered fluid flowsonly along the paths described herein; these seals are or may beconventional and thus will not be described in detail.

The exemplary main body assembly 120 comprises a main body member 140and, optionally, a variable orifice assembly 142. The main body member140 defines at least a portion of the main passageway 130, the inlet132, and the outlet 134. The main body member comprises an inlet section144, an outlet section 146, and a intermediate section 148.

The main body member 140 further defines first and second balancingports 150 and 152 located upstream and downstream, respectively, of thevariable orifice assembly 142. The first and second balancing ports 150and 152 allow fluid communication between the pressure balancing system24 and the inlet and outlet sections 144 and 146, respectively, of themain passageway 130. The first balancing port 150 and second balancingport 152 are connected to input and output ports 154 and 156,respectively, of the pressure balancing system 34.

The exemplary pressure and temperature sensors 40 and 42 used by themeter system 20 are arranged to detect the pressure and temperature ofthe metered fluid flowing through the main passageway 130. Inparticular, the main body member 140 defines first and second test ports160 and 162 arranged in the outlet section 146 of the main body member140. The test ports 160 and 162 may, however, be arranged in the inletand/or intermediate sections 144 or 148 of the body member 140 inanother embodiment of the present invention.

The sensors 40 and 42 are or may be conventional and are inserted orthreaded into the test ports 160 and 162. Seals are conventionallyformed between the sensors 40 and 42 and the test ports 160 and 162. Soattached to the main body member 140, the sensors 40 and 42 generateelectrical pressure and temperature signals that correspond to thepressure and temperature of the metered fluid immediately adjacent tothe test ports 160 and 162.

The inlet, outlet, and restriction sections 144, 146, and 148 of themain body member 140 serve different functions and thus have differentgeometries. The inlet and outlet sections 144 and 146 are threaded orotherwise adapted to allow a fluid-tight connection to be made betweenthe main body member 140 and the source 54 and destination 56 of themetered fluid. The effective cross-sectional areas of inlet and outletsections 144 and 146 are not crucial to any implementation of thepresent invention except that the flow of metered fluid to the fluiddestination 56 must meet predetermined system requirements. In theexemplary main body assembly 120, the inlet and outlet sections 144 and146 define cylindrical inlet and outlet internal wall surfaces 170 and172 and have substantially the same diameter and effectivecross-sectional area.

The intermediate section 148 of the main body member 140 serves torestrict the flow of metered fluid through the main passageway 130 whilestill allowing the flow of metered fluid to meet the systemrequirements. The effective cross-sectional area of at least a portionof the intermediate section 148 of the main passageway 130 is thussmaller than that of the inlet and outlet sections 144 and 146. Inparticular, the intermediate section 148 is defined at least in part byan internal restriction wall 180 of the main body member 140. Therestriction wall 180 is substantially cylindrical and has a diametersmaller than that of the inlet and outlet wall surfaces 170 and 172.

The meter system 20 of the present invention may be manufactured withoutthe optional variable orifice assembly 142. In this case, therestriction wall 180 of the main body member 140 defines the restrictionchamber 32. The main body member 40 must be manufactured to tighttolerances and/or the calibration data may need to be calculated foreach main body member 140 to account for variations in the restrictionportions defined by individual main body members if a variable orificeassembly is not used.

If a variable orifice assembly 142 is used, the restriction chamber 32associated with a given main body member 140 may be altered to calibratethe given main body member 140. Any number of mechanisms may be used toalter the geometry of the restriction chamber 32.

In the meter system 20, the exemplary variable orifice assembly 142comprises a tube member 220 having an internal surface 222. The internalsurface 222 of the tube member 220 defines the effective cross-sectionalarea of the restriction chamber 32.

In some situations, the tube member 220 may be made of a rigid materialsuch as some metals or polymers. In this case, the tube member 220 ismade in a plurality of predetermined configurations each correspondingto a restriction chamber 32 having a different predeterminedcross-sectional area. One of these predetermined configurations isselected to obtain a desired geometry of the restriction chamber 32.

The exemplary tube member 220 is, however, made of a deformable materialsuch that, when the tube member 220 is deformed, the effectivecross-sectional area of the restriction chamber 32 is changed. Theexemplary tube member 220 is made of metal, but polymers, naturalrubber, or other materials may be used depending upon the circumstances.In this respect, the tube member 220 may be made of elastic (e.g.,polymers or natural rubber) or non-elastic (e.g., metal) material.

The variable orifice assembly 142 used by the exemplary meter system 20further comprises a compression wedge 224, a compression shim 226, firstand second chevron members 228 and 230, and a compression nut 232 havinga threaded surface 234.

To accommodate this variable orifice assembly 142, the intermediatesection 148 of the exemplary main body member 140 comprises thefollowing interior walls in addition to the restriction wall 180: a tubeseat wall 240, a compression wall 242, a spacing wall 244, and athreaded wall 246. The tube seat wall 240 is located upstream of therestriction wall 180 described above and is generally cylindrical. Thecompression wall 242 is located upstream of the tube seat wall and isgenerally conical. The spacing wall 244 is located upstream of thecompression wall and is generally cylindrical. The threaded wall 246 islocated upstream of the spacing wall and is threaded to mate with thethreaded surface 232 of the compression nut 230.

Axial rotation of the compression nut 230 relative to the body member140 thus causes the nut 230 to be displaced along a longitudinal axis Aof the body member 140 towards the restriction wall 180. As the nut 230moves towards the restriction wall 180, the nut 230 applies a force onthe compression wedge 224 through the chevron members 228 and 230 andcompression shim 226. The compression wedge 224 comprises a conicalouter surface 250. The outer surface 250 of the compression wedge 224engages the compression wall 242 such that the wedge 224 moves radiallyinwardly towards the longitudinal axis A. The inward movement of thecompression wedge 224 deforms, as generally described above, the tubemember 220 to alter the effective cross-sectional area of therestriction chamber 32.

III. Electrical System

Referring now to FIG. 4 of the drawing, depicted in detail therein isone exemplary embodiment of a meter circuit 44 used as part of theelectrical system 24 of the exemplary flow meter system 20. The metercircuit 44 comprises first, second, and third summing and scalingsystems 320, 322, and 324. The first summing and scaling system 320combines the calibration factor and raw pressure signal to obtain acalibrated pressure signal. The second summing and scaling system 322combines the raw temperature signal and the gas constant input to obtaina compensated temperature signal. The third summing and scaling system324 combines the calibrated pressure signal and the compensatedtemperature signal to obtain the flow output signal.

The design details of the summing and scaling systems 320-324 will bedetermined by the specific environment in which the meter system 20 isto be used. Typically, these systems 320-324 will comprise signalspecific components and a summing and scaling amplifier. The signalspecific components convert a raw input signal in either analog ordigital form into a digital or analog conditioned signal suitable foruse by the summing and scaling amplifier associated with the signalspecific components. The summing and scaling amplifier in turn isdesigned to generate a scaled signal based on the conditioned inputsignals.

The meter circuit 44 may be implemented using discrete circuitcomponents, an application specific integrated circuit (ASIC), softwarerunning on an integrated processor such as a general purposemicrocomputer or a digital signal processor, or a combination of thesemethods. The exact nature of any given implementation the electricalsystem 24 will depend upon such factors as manufacturing costs, thedesigners background and experience, and the operating environment ofthe meter system 20. For example, in an embodiment of the presentinvention implemented with a digital signal processor (“DSP”), the DSPpreferably comprises a memory unit having look-up tables that storecalibration conditions including but not limited to the originalcalibration conditions for the meter. This data is useful for referenceback to original conditions in the case of pressure and/or temperaturesensor drift. The data is also useful for conducting diagnosticprocedures to determine whether the meter requires calibration or otherservice. Additionally, the DSP memory unit preferably has a look-uptable of fluid viscosity vs. temperature for one or more fluids. Thisdata is useful for use in compensating for changes in fluid temperature.

Referring now to FIG. 5, depicted therein is one exemplary meter circuit44 adapted to generate the flow output signal based on analog inputsignals. As shown in FIG. 5, the first summing and scaling system 320comprises a signal conditioning module 330, an optional arithmetic logicunit 332, an optional linearization amplifier 334, and a first andsecond summing and scaling amplifiers 336 and 338.

The raw pressure signal is initially filtered and amplified by thesignal conditioning module 330. If necessary, the filtered pressuresignal is then applied to one or both of the arithmetic logic unit 332and linearization amplifier 334 and then to the first summing andscaling amplifier 336. If the arithmetic logic unit 332 andlinearization amplifier 334 are not required, the filtered pressuresignal is directly passed to the first summing and scaling amplifier336. The second summing and scaling amplifier 338 generates acalibration signal based on the calibration factor. The pressure signaland calibration signal are then applied to the first summing and scalingamplifier 336 to obtain the processed pressure signal.

The second summing and scaling system 322 comprises a signalconditioning module 340, a scaling and gain amplifier 342, and a summingand scaling amplifier 344. The signal conditioning module 340 filtersand amplifies the temperature signal to obtain a filtered temperaturesignal. The scaling and gain amplifier 342 generates a gas constantsignal based on the gas constant input. The summing and scalingamplifier 344 generates the processed temperature signal based on thefiltered temperature signal and the gas constant signal.

The third summing a scaling system 324 comprises a summing and scalingamplifier 350 and a buffer amplifier 352. The summing and scalingamplifier generates a flow signal based on the calibrated pressuresignal and the compensated temperature signal as generally describedabove. The buffer amplifier 352 generates the flow output signal basedon the flow signal.

IV. Calibration Process

Referring now to FIG. 5 of the drawing, depicted therein at 360 is aflow diagram of one exemplary process for calibrating the meter system20 described above. In the following discussion, the particular metersystem 20 being calibrated will be referred to as the DUT.

The first step 362 of the calibration process is to connect the flowrestrictor 30 of the DUT in series with a calibrated meter system. Anegative gauge pressure or vacuum is then applied at step 364 to theoutlet of the flow restrictor 30 of the DUT, and the electronics of themeter system 20 are set to zero.

The next step 366 is to apply pressure upstream of the DUT to createflow through the DUT. The flow is measured using the calibrated metersystem. The gas specific gas constant input is then applied at step 368to the electronic portion 24 using conventional means such as a digitalserial input and/or a set of one or more switches that can be configuredto generate the appropriate gas constant input.

The maximum flow range is then obtained at step 370 by selecting anappropriate geometry of the restriction cavity 32 by any one of themethods described above.

The flow is then decreased at step 372 to ten percent of the maximumflow setting of the DUT. The pressure and temperature signals associatedwith that flow are then read and stored. At step 374, the flow rate isincreased in increments of ten percent up to one hundred percent. Thepressure and temperature signals associated with each incrementalincrease in flow rate are measured and stored. The slope of plot of thepressure signal versus the mass flow rate measured at step 378 by thecalibrated meter system is measured and stored as the calibration factorusing conventional means such as a trim pot or digital serial input.

Referring to FIG. 6, depicted at 380 a, 380 b, and 380 c therein areexemplary plots of pressure signal versus mass flow rate for severaltemperatures. The meter circuit 44 generates the flow signal outputsignal based on the pressure/mass flow plots created by the calibrationfactor and gas constant input.

Referring now to FIG. 7, depicted therein is a plot 382 of the pressuresignal versus mass flow rate in which the relationship between thepressure signal and mass flow rate is non-linear. For example, thisrelationship may be non-linear in the case of an orifice.

If the pressure/mass flow rate relationship is non-linear, the filteredpressure signal will be passed through one or both of the arithmeticlogic unit 332 and linearization amplifier 334. The arithmetic logicunit 332 and linearization amplifier 334 implement a function thatcompensates for the non-linearity of the pressure/mass flow raterelationship. For example, the signal conditioning circuitry may performone or both of a “piecewise linearization” function or a square rootfunction on the filtered pressure signal to obtain the compensatedpressure signal. In particular, referring back to FIG. 8, depicted at384 is a curve corresponding to the inverse of the non-linear curve 382.A curve 386 represents the midpoint of the curves 382 and 384 and can beused in the linear slope equation (10) described above.

In practice, the meter circuit 44 is preferably manufactured with boththe arithmetic logic unit 332 and linearization amplifier 334 and, asshown in FIG. 5, switches 390 and 392 configured to allow either ofthese circuit elements 332 and 334 to be removed from the circuit 44.The use of the switches 390 and 392 thus allows the production of astandard meter circuit 44 that can easily be customized for a particularenvironment.

V. Mass Flow Control System

As generally described above, the mass flow meter of the presentinvention described above has numerous applications. It can be usedalone simply to measure mass flow rate of a wide variety of fluids at awide variety of flow rates. It can be used as part of a larger system ofprocessing or administering fluids where accurate mass flow rates areimportant. It can also be combined with other components to obtain amore complex stand alone device.

Described in this section with reference to FIG. 9 is an exemplary massflow control system 420 that incorporates the exemplary mass flow meter20 described above. The mass flow control system 420 is a stand alonedevice that not only measures mass flow rate but allows this flow rateto be controlled with a high degree of accuracy for a wide variety offluids and flow rates.

The mass flow control system 420 incorporates the flow meter system 20described above, and the meter portion of the flow control system 420will not be described again except to the extent necessary for acomplete understanding of the flow control system 420.

In addition to the flow meter system 20, the flow control system 420comprises a valve control feedback loop system 422 and a flow controllersystem 424. The flow controller system 424 is arranged in series withthe flow meter system 20 such that the flow controller system 424determines the mass flow of fluid through the flow meter system 20.

Preferably, the flow controller system 420 is a mechanical orelectro-mechanical flow controller such as is described in the '849patent and '708 application cited above. The flow controller system 424may, however, be any flow controller system that can increase ordecrease the flow of fluid through the system 420 under electrical ormechanical control.

In the present invention, the flow signal generated by the summing andscaling amplifier 350 of the third summing and scaling system 324 isapplied to the valve control feedback loop system 422. The valve controlfeedback loop system 422 compares the flow signal with a desired flowrate signal. The desired flow rate signal may be preset or may bechanged as required by the circumstances. For example, in a medicalsetting, a doctor may prescribe that a gas be applied to a patient at apredetermined flow rate. The predetermined flow rate determined by thedoctor would be converted into the desired flow rate signal.

Based on the difference between the desired flow rate signal and theflow signal generated by the flow meter system 20, the valve controlfeedback loop system 422 generates a flow control signal that controlsthe flow controller system 424. If the flow controller system 424 is amechanical system, the flow control signal will be in the form ofmechanical movement (rotational, translational) that operates the flowcontroller signal to increase or decrease the fluid flow rate throughthe system 424. If the system 424 is an electro-mechanical system, theflow control signal may take the form of an electrical signal that isconverted to mechanical movement at the system 424.

The combination of the flow controller system 424 and the flow metersystem 20 results in the fluid output of the system 420 beingcontrollable to a high degree of accuracy.

VI. Alternative Embodiment of Mass Flow Control System

An alternative embodiment of the mass flow control system describedabove and in FIG. 9 is shown in FIG. 10. Specifically, the flowcontroller 524 comprises either a piezoelectric actuator control or asolenoid actuator control 526 coupled to a valve 528. The actuatorcontrol 526 delivers a signal 530 to the meter circuit 44. If theactuator control 526 is a solenoid actuator control then the signal 530is a current signal is converted to a voltage signal. If the actuatorcontrol 526 is a piezoelectric actuator control, the signal 530 is avoltage signal from an integrated strain gauge. In either instance, thesignal 530 can be identified as V_(pm), i.e., Voltage (prime mover).

The signal 530 represents the relationship between the Lorentz forcegenerated in the actuator control 526 by changes in pressure in thevalve 528. Hence, the signal 530 can be used as an indirect pressureindicator replacing, augmenting and/or calibrating the pressuretransducer 40 in FIG. 1. For example, in FIG. 10, the pressuretransducer 40 is not present and the signal 530 is used in its stead.The signal 530 can also be used as a diagnostic indicator to verify thatthe value of R for a given flow restriction has not changed.

Preferably, the meter circuit 44 for the mass flow control system shownin FIG. 10 has at least 128 kilobytes of memory using EEPROM. The memoryfor the meter circuit 44 should contain a look up table of values ofV_(pm) for incremental mass flow rates for various gases and/or flowrestrictions. This look-up table preferably represents values for theequation: $\begin{matrix}{\overset{.}{m} = \frac{K\quad V_{p\quad m}}{R\quad T}} & (11)\end{matrix}$

Thus, an alternative embodiment using a signal 530, V_(pm), to measurechanges in pressure in the system and to control the valve 528 in themass flow control system is described above.

VII. Additional Considerations

A designer will typically design a particular implementation of thepresent invention by initially determining the operating environment inwhich the flow meter system is to be used. The operating environmentwill include the properties of the fluid itself, the expected range offluid input and output pressures, the ambient conditions, the tolerancefor error, and the like. The designer may also consider commercialfactors such as cost.

The properties of many of both the mechanical and electrical componentsof the present invention will be changed depending upon thecircumstances to “tune” a specific flow meter system for a particularuse.

For example, the restriction chamber and inlet and outlet openings maybe selected based on the type of fluid, expected inlet pressures, anddesired flow rates.

In addition, the materials used for the various components must beselected based on the pressures and types of fluids expected. Forexample, for air at low pressures, plastic may be used for many of thecomponents. For caustic fluids and higher pressures, steel or stainlesssteel may be used.

The electronics will also be customized for a particular environment.For example, the implementation details of the various summing andscaling systems described above will be determined once the particularoperating environment is defined.

Accordingly, the present invention may be embodied in forms other thanthose described herein without departing from the spirit or essentialcharacteristics of the invention. The present embodiments are thereforeto be considered in all respects as illustrative and not restrictive,the scope the invention being indicated by the appended claims ratherthan by the foregoing description; and all changes which come within themeaning and range of equivalency of the claims are therefore intended tobe embraced therein.

What is claimed is:
 1. A method for calibrating a flow meter; said flowmeter comprising an inlet with a diameter and an outlet with a diameter;a flow restrictor having a restriction chamber and a pressure balancingsystem interposed between the inlet and outlet; the restriction chamberhaving a variable orifice assembly; a cylindrical restriction wall witha diameter less than the diameters of the inlet and outlet; and, apressure sensor and a temperature sensor upstream from the flowrestrictor providing input to a meter circuit having calibration data;whereby the meter circuit generates a flow output signal based on thecalibration data and the input from the pressure sensor and temperaturesensor; the steps of said method comprising: connecting the flow meterto a referenced flow standard; applying a negative gauge pressure to theoutlet; setting a zero point on the meter circuit; applying pressureupstream of the meter; measuring a flow; entering a gas specificconstant into the calibration data of the meter circuit; obtaining amaximum flow range by adjusting the variable orifice assembly for agiven inlet pressure; decreasing the flow to ten percent of maximumflow; measuring the pressure and temperature and storing results in themeter circuit; increasing the flow by ten percent increments up to themaximum flow; measuring the pressure and temperature at each ten percentincrement and storing the results in the meter circuit; calculating aslope of a graph of pressure versus mass flow rate from the resultsstored in the meter circuit; setting the meter circuit by using theslope.
 2. The method of claim 1 where the meter circuit furthercomprises an arithmetic logic unit and a linearization amplifier and,where the graph is non-linear, the steps of said method furthercomprise: performing a piecewise linearization function on a filteredpressure signal to obtain a compensated pressure signal for use insetting the meter circuit.
 3. The method of claim 1 where the methodfurther comprises the following step: measuring a current and storingresults in the meter circuit as a pressure.